Method for preparing a 4-hydroxybenzaldehyde and derivatives

ABSTRACT

The present invention concerns a process for the preparation of a 4-hydroxybenzaldehyde and its derivatives. More particularly, the invention concerns the preparation of 3-methoxy-4-hydroxybenzaldehyde and 3-ethoxy-4-hydroxybenzaldehyde, respectively known as vanillin and ethylvanillin. The process for the preparation of a 4-hydroxybenzaldehyde and its derivatives is characterized in that it consists of selectively oxidising, to a carboxy group, the group in the 2 position with respect to the hydroxyl group, present in the phenolic compounds in a mixture comprising at least a phenolic compound (A) carrying formyl and/or hydroxymethyl groups in the 2 and 4 position, a phenolic compound (B) carrying a formyl or hydroxymethyl group in the 4 position, a phenolic compound (C) carrying a formyl or hydroxymethyl group in the 2 position, resulting in a mixture comprising a 3-carboxy-4-hydroxybenzaldehyde, a 4-hydroxybenzaldehyde and a 2-hydroxybenzoic acid, which then undergoes a decarboxylation operation to produce the 4-hydroxybenzaldehyde and a phenol which can optionally be recycled.

This application is an application under 35 U.S.C. Section 371 ofInternational Application Number PCT/FR97/02085, filed on Nov. 19, 1997.

The present invention concerns a process for the preparation of a4-hydroxybenzaldehyde and its derivatives.

More particularly, the invention concerns the preparation of3-methoxy-4-hydroxybenzaldehyde and 3-ethoxy-4-hydroxybenzaldehyde,respectively known as vanillin and ethylvanillin.

In French patent application n^(o) 95/06186, a process is described forthe preparation of 4-hydroxybenzaldehydes, more particularly vanillinand ethylvanillin.

The process described consists of preparing a3-carboxy-4-hydroxybenzaldehyde then decarboxylating that compound toproduce the 4-hydroxybenzaldehyde.

According to FR n^(o)95/06186, the 3-carboxy-4-hydroxybenzaldehyde isprepared from one of the compounds given below and mixtures thereof,more particularly with the following formulae (IIa), (IIb), (IIc) and(IId) given below:

where:

M represents a hydrogen atom and/or a metallic cation from group (Ia) or(IIa), or an ammonium cation;

Z₁, Z₂ and Z₃, which may be identical or different, represent a hydrogenatom, an alkyl, alkenyl, alkoxy, hydroxyalkyl, alkoxyalkyl, cycloalkylor aryl radical, a hydroxyl group, a nitro group, a halogen atom, or atrifluoroethyl group.

In that patented process, the starting compound is a bifunctionalphenolic compound carrying two functional groups on the aromatic ring inthe ortho and para positions which can be a —CHO group and/or a —CH₂OHgroup.

Firstly, the ortho group is selectively oxidised to a carboxylic group;the group in the para position is at most oxidised to a formyl group.After eliminating the carboxy group in the ortho position, a4-hydroxybenzaldehyde is obtained.

Thus vanillin and ethylvanillin can advantageously be prepared using aprocess which is selective and also highly competitive from anindustrial viewpoint as it uses inexpensive reactants.

However, in that process it is difficult to obtain a reaction yield(expressed with respect to the starting phenol) of more than 70% asobtaining a high yield of a bifunctional phenolic compound isaccompanied by that of a by-product, namely a bis-arylmethane.

During our research, we discovered in French application no96/12479 thata 4-hydroxybenzaldehyde can be prepared from a mixture ofmonosubstituted phenolic compounds, one (A) carrying a formyl orhydroxymethyl group in the 2 position, and the other (B) carrying aformyl or hydroxymethyl group in the 4 position, and selectivelyoxidising the formyl or hydroxymethyl group in the 2 position ofcompound (A) to a carboxy group, and possibly a hydroxymethyl group inthe 4 position of compound (B) to a formyl group, thus producing amixture of a 2-hydroxybenzoic acid and a 4-hydroxybenzaldehyde fromwhich the latter is separated.

More particularly, that mixture of phenolic compounds used has generalformula (II):

where, in formulae (IIA) and (IIB):

Y₁ and Y₂, which may be identical or different, represent one of thefollowing groups:

a —CHO group;

a CH₂OH group;

Z₁, Z₂ and Z₃, which may be identical or different, represent a hydrogenatom, an alkyl, alkenyl, alkoxy, hydroxyalkyl, alkoxyalkyl, cycloalkylor aryl radical, a hydroxyl group, a nitro group, a halogen atom, or atrifluoroethyl group.

The disadvantage of that process is that to obtain compounds withformula (IIA) or (IIB), by hydroxymethylation of a phenol, it isessential to work with a low degree of conversion of the starting phenolwhich results in low productivity.

Thus the existing processes must be improved in order to have availablea process which is of great economic interest which minimises theby-products and can produce a high operating productivity.

We have discovered, and this constitutes one aspect of the presentinvention, a process for the preparation of a 4-hydroxybenzaldehyde andits derivatives, characterized in that it consists of selectivelyoxidising, to a carboxy group, the group in the 2 position with respectto the hydroxyl group, present in the phenolic compounds in a mixturecomprising at least:

a phenolic compound (A) carrying formyl and/or hydroxymethyl groups inthe 2 and 4 position;

a phenolic compound (B) carrying a formyl or hydroxymethyl group in the4 position;

a phenolic compound (C) carrying a formyl or hydroxymethyl group in the2 position;

resulting in a mixture comprising a 3-carboxy-4-hydroxybenzaldehyde, a4-hydroxybenzaldehyde and a 2-hydroxybenzoic acid, which then undergoesa decarboxylation operation to produce the 4-hydroxybenzaldehyde and aphenol which can optionally be recycled.

In a further aspect, the invention provides a starting mixture ofphenolic compounds claimed as the mixture itself, and the mixtureobtained after oxidation.

Finally, processes for preparing these mixtures constitute furtheraspects of the invention.

In the process of the invention, we have discovered that by startingfrom a mixture of starting compounds as defined above, it is possible tocarry out simultaneous intramolecular oxidation (A) and intermolecularoxidation (B+C) since oxidation of the carboxy group takes placepreferentially on the hydroxymethyl or formyl group in the orthoposition.

The process of the invention thus comprises an oxidation step and adecarboxylation step for a 3-carboxy-4-hydroxybenzaldehyde to a4-hydroxybenzaldehyde and a 2-hydroxybenzoic acid which can produce thephenolic starting compound which can then be recycled; the4-hydroxybenzaldehyde is then recovered conventionally.

The starting substrates used in the process of the invention aremixtures of phenolic compounds, one (A) carrying formyl and/orhydroxymethyl groups in the 2 and 4 positions, the second (B) carrying aformyl or a hydroxymethyl group in the 4 position and the last, (C), inthe 2 position.

The term “phenolic compound” means any aromatic compound with anaromatic nucleus which carries a hydroxy group.

In the following disclosure of the present invention, the term“aromatic” means the conventional idea of aromaticity as defined in theliterature, in particular in “Advanced Organic Chemistry” by JerryMARCH, 4^(th) edition, John Wiley and Sons, 1992, pp. 40 ff.

Thus a mixture (II) of phenolic compounds is used which, moreparticularly, have the following formulae:

In formulae (IIA) to (IIC):

Y₁ and Y₂, which may be identical or different, represent one of thefollowing groups:

a —CHO group;

a —CH₂OH group;

Z₁, Z₂ and Z₃, which may be identical or different, represent a hydrogenatom, an alkyl, alkenyl, alkoxy, hydroxyalkyl, alkoxyalkyl, cycloalkylor aryl radical, a hydroxy group, a nitro group, a halogen atom, or atrifluoromethyl group.

Particularly suitable compounds for use in the process of the inventionhave formulae (IIA) to (IIC) where Z₁, Z₂ and Z₃, which may be identicalor different, represent one of the following groups:

a hydrogen atom;

a linear or branched alkyl radical containing 1 to 12 carbon atoms,preferably 1 to 4 carbon atoms, such as methyl, ethyl, propyl,isopropyl, butyl, isobutyl, sec-butyl or tert-butyl;

a linear or branched alkenyl radical containing 2 to 12 carbon atoms,preferably 2 to 4 carbon atoms, such as vinyl or allyl;

a linear or branched alkoxy radical containing 1 to 12 carbon atoms,preferably 1 to 4 carbon atoms, such as a methoxy, ethoxy, propoxy,isopropoxy, butoxy, isobutoxy, sec-butoxy or tert-butoxy radical;

a phenyl radical;

a halogen atom, preferably a fluorine, chlorine or bromine atom.

The present invention does not exclude the presence of substituents ofdifferent natures on the aromatic ring, provided that they do notinterfere with the reactions taking place in the process of theinvention.

The present invention is preferably applicable to compounds with formula(IIA) to (IIC) where Z₁ represents a hydrogen atom or a linear orbranched alkyl or alkoxy radical containing 1 to 6 carbon atoms,preferably 1 to 4 carbon atoms; Z₂ and Z₃ represent a hydrogen atom; andY₁ and Y₂ are identical and represent a formyl group or a hydroxymethylgroup.

Preferred examples of mixtures of phenolic compounds for use in theprocess of the invention are:

o-hydroxymethylguaiacol, p-hydroxymethylguaiacol and4,6-di(hydroxymethyl)guaiacol;

o-formylguaiacol, p-formylguaiacol and 4,6-diformylguaiacol;

o-hydroxymethylguetol, p-hydroxymethylguetol and4,6-di(hydroxymethyl)guetol;

o-formylguetol, p-formylguetol and 4,6-diformylguetol.

The process of the invention uses a starting mixture of phenoliccompounds which preferably have formula (II).

The proportion of each phenolic compound in the mixture depends on themethod of their preparation.

Preferred mixtures comprise:

30% to 70% by weight, more preferably 50% to 70%, of a phenolic compound(A);

30% to 70% by weight, more preferably 50% to 70%, of a mixture ofphenolic compounds (B+C).

As an indication, the quantities of isomers B and C are approximatelyequimolar in the mixture.

A reaction scheme is given below to facilitate comprehension of thedisclosure of the invention without in any way limiting the scope of theinvention to the scheme.

In formulae (I) to (VI):

Y₁ and Y₂, which may be identical or different, represent one of thefollowing groups:

a —CHO group;

a —CH₂OH group;

M represents a hydrogen atom and/or a metallic cation from group (Ia) or(IIa) of the periodic table, or an ammonium cation;

Z₁, Z₂ and Z₃, have the meanings given above.

In the present text, reference will be made to the periodic tablepublished in the “Bulletin de la Société Chimique de France”, n^(o)1(1966).

In accordance with the process of the invention, the Y₁ group inposition 2 of phenolic compounds (A) and (C) preferably with formulae(IIA) and (IIC) is selectively oxidised to a carboxy group, and ahydroxymethyl group, if present in the 4 position in phenolic compounds(A) and (B) preferably with formulae (IIA) and (IIB), is selectivelyoxidised to a formyl group.

Oxidation is carried out using molecular oxygen or a gas containingmolecular oxygen, generally in the presence of a catalyst.

A preferred oxidation method consists of oxidising a mixture of phenoliccompounds with formula (II) in the liquid phase using molecular oxygenor a gas containing molecular oxygen, in an aqueous medium comprising abasic agent, in the presence of a catalyst based on a metal M₁ selectedfrom metals from group 1b and 8 of the periodic classification of theelements, which may optionally contain, as an activator, metals such ascadmium, cerium, bismuth, lead, silver, tellurium or tin.

We have surprisingly discovered that if the temperature is increased andthe reaction is preferably carried out under pressure or if the quantityof base present during oxidation is increased, group Y₁ in the 2position in phenolic compounds (A) and (C), preferably with formulae(IIA) and (IIC) is selectively oxidised to a carboxy group, and thegroup located in the 4 position in phenolic compounds (A) and (B),preferably with formulae (IIA) and (IIB), is at most oxidised to theformyl group.

The catalysts used in the process of the invention are based on a metalfrom group 1b and 8 of the periodic classification.

Examples of catalysts based on a metal from group 8 of the periodicclassification are nickel, ruthenium, rhodium, palladium, osmium,iridium, platinum and mixtures thereof. Regarding metals from group 1b,copper is preferred.

Preferably, platinum and/or palladium catalysts are used, in anyavailable form such as: platinum black, palladium black, platinum oxide,palladium oxide or the noble metal itself deposited on differentsupports such as carbon black, calcium carbonate, aluminas or activatedsilicas or equivalent materials. Catalytic masses based on carbon blackare particularly suitable.

The quantity of catalyst used, expressed as the weight of metal M₁ withrespect to that of the phenolic compound with formula (II), can varyfrom 0.01% to 10%, preferably 0.04% to 2%.

Further details of the catalysts can be obtained from U.S. Pat. No.3,673,257, and French patents FR-A-2 305 420 and FR-A2 350 323.

The activator can be selected from all those mentioned in the abovepatents. Preferably, bismuth, lead and cadmium are used as the freemetal or as cations. In the latter case, the associated anion is notcritical and all derivatives of these metals can be used. Preferably,bismuth metal or its derivatives is used.

An inorganic or organic bismuth derivative can be used in which thebismuth atom has an oxidation number of more than zero, for example 2,3, 4 or 5. The residue associated with the bismuth is not criticalprovided that is satisfies this condition. The activator can be solubleor insoluble in the reaction medium.

Illustrative examples of activators which can be used in the process ofthe present invention are: bismuth oxides; bismuth hydroxides; salts ofinorganic hydracids such as: bismuth chloride, bromide, iodide,sulphide, selenide, or telluride; salts of inorganic oxyacids such as:bismuth sulphite, sulphate, nitrite, nitrate, phosphite, phosphate,pyrophosphate, carbonate, perchlorate, antimonate, arsenate, selenite,or selenate; and salts of oxyacids derived from transition metals suchas: bismuth vanadate, niobate, tantalate, chromate, molybdate,tungstate, or permanganate.

Other suitable compounds are the salts of aliphatic or aromatic organicacids such as: bismuth acetate, propionate, benzoate, salicylate,oxalate, tartrate, lactate, or citrate; and phenates such as: bismuthgallate or pyrogallate. These salts and phenates can also be bismuthylsalts.

Other inorganic or organic compounds are binary compounds of bismuthwith elements such as phosphorous or arsenic; heteropolyacids containingbismuth and salts thereof; also aliphatic and aromatic bismuthines.

Specific examples are:

oxides: BiO; Bi₂O₃; Bi₂O₄; Bi₂O₅;

hydroxides: Bi(OH)₃;

salts of inorganic hydracids: bismuth chloride BiCl₃; bismuth bromideBiBr₃; bismuth iodide BiI₃; bismuth sulphide Bi₂S₃; bismuth selenideBi₂Se₃; bismuth telluride Bi₂Te₃;

salts of inorganic oxyacids: basic bismuth sulphiteBi₂(SO₃)₃,Bi₂O₃,5H₂O; neutral bismuth sulphate Bi₂(SO₄)₃; bismuthylsulphate (BiO)HSO₄; bismuthyl nitrite (BiO)NO₂,0.5H₂O; neutral bismuthnitrate Bi(NO₃)₃,5H₂O; double nitrate of bismuth and magnesium2Bi(NO₃)₃,3Mg(NO₃)₂,24H₂O; bismuthyl nitrate (BiO)NO₃; bismuth phosphiteBi₂(PO₃H)₃,3H₂O; neutral bismuth phosphate BiPO₄; bismuth pyrophosphateBi₄(P₂O₇)₃; bismuthyl carbonate (BiO)₂CO₃;0.5H₂O; neutral bismuthperchlorate Bi(ClO₄)₃,5H₂O; bismuthyl perchlorate (BiO)ClO₄; bismuthantimonate BiSbO₄; neutral bismuth arsenate Bi(AsO₄)₃; bismuthylarsenate (BiO)AsO₄,5H₂O; bismuth selenite Bi₂(SeO₃)₃;

salts of oxyacids derived from transition metals: bismuth vanadateBiVO₄; bismuth niobate BiNbO₄; bismuth tantalate BiTaO₄; neutral bismuthchromate Bi₂(CrO₄); bismuthyl dichromate ([BiO]₂Cr₂O₇; acid bismuthylchromate H(BiO)CrO₄; double chromate of bismuthyl and potassiumK(BiO)CrO₄; bismuth molybdate Bi₂(MoO₄)₃; bismuth tungstate Bi₂(WO₄)₃;double molybdate of bismuth and sodium NaBi(MoO₄)₂; basic bismuthpermanganate Bi₂O₂(OH)MnO₄;

salts of aliphatic or aromatic organic acids: bismuth acetateBi(C₂H₃O₂)₃; bismuthyl propionate (BiO)C₃H₅O₂; basic bismuth benzoateC₆H₅CO₂Bi(OH)₂; bismuthyl salicylate C₆H₄CO₂(BiO)(OH); bismuth oxalate(C₂O₄)₃Bi₂; bismuth tartrate Bi₂(C₄H₄O₆)₃,6H₂O; bismuth lactate(C₆H₉O₅)OBi,7H₂O; bismuth citrate C₆H₅O₇Bi;

phenates: basic bismuth gallate C₇H₇O₇Bi; basic bismuth pyrogallateC₆H₃(OH)₂(OBi)(OH).

Other inorganic or organic compounds are also suitable: bismuthphosphide BiP; bismuth arsenide Bi₃As₄; sodium bismuthate NaBiO₃;bismuth-thiocyanic acids H₂[Bi(BNS)₅],H₃[Bi(CNS)₆] and sodium andpotassium salts thereof; trimethylbismuthine Bi(CH₃)₃,triphenylbismuthine Bi(C₆H₅)₃.

Preferred bismuth derivatives for use in the process of the inventionare: bismuth oxides; bismuth hydroxides; bismuth or bismuthyl salts ofinorganic hydracids; bismuth or bismuthyl salts of inorganic oxyacids;bismuth or bismuthyl salts of aliphatic or aromatic organic acids; andbismuth or bismuthyl phenates.

A particularly suitable group of activators for carrying out the processof the invention is constituted by: bismuth oxides Bi₂O₃ and Bi₂O₄;bismuth hydroxide Bi(OH)₃; neutral bismuth sulphate Bi₂(SO₄)₃; bismuthchloride BiCl₃; bismuth bromide BiBr₃; bismuth iodide BiI₃; neutralbismuth nitrate Bi(NO₃)₃,5H₂O; bismuthyl nitrate BiO(NO₃); bismuthylcarbonate (BiO)₂CO₃,0.5H₂O; bismuth acetate Bi(C₂H₃O₂)₃; and bismuthylsalicylate C₆H₄CO₂(BiO)(OH).

The quantity of activator used, expressed as the quantity of metalcontained in the activator with respect to the weight of metal M₁ used,can be varied between wide limits. As an example, this quantity can beas low as 0.1% and can attain the weight of metal M₁ used, or evenexceed it without any problems.

More particularly, this quantity is selected so that it provides theoxidation medium with 10 ppm to 900 ppm by weight of activator metalwith respect to the phenolic compound with formula (II). In thisrespect, higher quantities of activator of the order of 900 ppm to 1500ppm can naturally be used, but with no great additional advantage.

In the process of the invention, oxidation is carried out in an aqueousmedium containing a basic agent in solution, more particularly ammoniumhydroxide, alkaline or alkaline-earth bases, for example hydroxides suchas sodium, potassium, lithium and baryte hydroxides; alkalinealkanolates such as sodium or potassium methylate, ethylate,isopropylate or t-butylate, sodium or potassium carbonates orbicarbonates and in general, the salts of alkaline or alkaline-earthbases and weak acids.

Thus compounds with formulae (III) to (V) can be completely or partiallyin their salt form depending on the quantity of basic agent used. Itfollows that in these formulae, M, the residue of the base, generallysymbolises a hydrogen atom and/or a metallic cation from group (Ia) or(IIa) or an ammonium cation.

Sodium or potassium hydroxide is used for reasons of economy. Theproportion of inorganic base to be used can be in the range 0.5 to 10moles, preferably in the range 1 to 4 moles, more preferably in therange 2 to 4 moles, of inorganic base per mole of phenolic compoundswith formula (II).

The concentration by weight of the mixture of phenolic compounds withformula (II) in the liquid phase is usually in the range 1% to 60%,preferably in the range 2% to 30%.

In practice, one implementation of the process consists of bringing thesolution comprising the mixture of phenolic compounds with formula (II),the basic agent, the catalyst based on metal M₁, and any activator intocontact with molecular oxygen or a gas containing molecular oxygen inthe proportions indicated above.

Atmospheric pressure can be used, but it is preferable to operate at apressure of 1 to 20 bar.

The mixture is then stirred at the desired temperature until a quantityof oxygen has been consumed which corresponds to that necessary totransform the hydroxymethyl group or formyl group of compounds (A) and(C) into a carboxy group, and the hydroxymethyl group, if present incompounds (A) and (B), into a formyl group.

The reaction temperature to be used depends on the thermal stability ofthe products to be prepared.

In accordance with the invention, the temperature is preferably selectedso as to be in the range 30° C. to 200° C., preferably in the range 40°C. to 160° C.

The temperature can be adapted to the reaction conditions by the skilledperson (in particular the quantity of base, nature of metal M₁, pressureand stirring). In particular, it has been discovered that the lower thetemperature the higher must be the quantity of basic agent used.

Examples of preferred conditions in the case of preferred metalsplatinum and palladium will now be given. For platinum, the temperaturecan be between 60° C. and 160° C., the quantity of base to be used isadvantageously in the range 1 to 3 moles per mole of phenolic compoundswith formula (II). For palladium, the temperature can be between 30° C.and 200° C., preferably between 30° C. and 150° C., and for the latterinterval, the quantity of base is preferably 2 to 4 moles per mole ofphenolic compounds.

The quantity of base must be sufficient to oxidise the Y₁ group in theposition ortho to the carboxy group. It is determined by the skilledperson depending on the temperature and the selected metal.

At the end of the reaction, which preferably takes 30 minutes to 6hours, a mixture comprising a 3-carboxy-4-hydroxybenzaldehyde preferablywith formula (III), a 4-hydroxybenzaldehyde preferably with formula (V)and a 2-hydroxybenzoic acid preferably with formula (IV) are recovered:the compounds can be partially or totally in their salt form.

After any necessary cooling, the catalytic mass and the reaction massare separated, for example by filtering.

In a second step of the process of the invention, the reaction mediumundergoes a decarboxylation reaction.

This is carried out by acidifying the resulting medium by adding aprotonic mineral acid, preferably hydrochloric acid or sulphuric acid oran organic acid such as trifluoromethanesulphonic acid ormethanesulphonic acid, to obtain a pH 3 or less.

The concentration of acid is immaterial and preferably, commerciallyavailable concentrations are used.

The reaction medium is heated to a temperature of between 120° C. and350° C., preferably between 150° C. and 220° C.

The process is preferably carried out under autogenous pressure of thereactants.

At the end of the reaction, the reaction medium is cooled to between 20°C. and 80° C.

A two-phase medium is obtained constituted by an organic phasecomprising the 4-hydroxybenzaldehyde preferably with formula (VI) andthe starting phenolic compound with formula (I), and also a salineaqueous phase.

The organic and aqueous phases are separated and the4-hydroxybenzaldehyde is recovered from the organic phase usingconventional separation techniques, for example extraction using anappropriate solvent (for example methylisobutylketone or isopropylether), or by distillation.

The improved process of the invention starts with a mixture of twophenolic compounds, one carrying a formyl or hydroxymethyl group in the2 and 4 position and the two others carrying a formyl or hydroxymethylgroup in the 2 or 4 position.

More particularly, the starting mixtures have the formulae given below:

in which formulae:

M represents a hydrogen atom and/or a metallic cation from group (Ia) orgroup (IIa), or an ammonium cation;

Z₁, Z₂, Z₃ have the meanings given above.

The mixtures of phenolic compounds, to which the process of theinvention can be applied, are prepared by a process which constitutes afurther aspect of the invention.

Thus mixtures of phenolic compounds with formulae (IIa₁) to (IIc₁) canbe obtained by a process for hydroxymethylating a phenol by condensingit with formaldehyde or a formaldehyde generator in an aqueous phase inthe presence of and alkaline or alkaline-earth base, so that the phenolconversion is at most 95% optionally followed by an oxidation step.

More precisely, a phenol is used which is not substituted in the orthoand para positions to the hydroxy group, with general formula (I):

where Z₁, Z₂ and Z₃ have the meanings given above.

Examples of phenols with formula (I) which can act as a starting pointfor the synthesis of compounds with formula (II) are guaiacol, guetol,3-methoxyphenol, 3-ethoxyphenol, 3-isopropoxyphenol, 3-t-butoxyphenol,m-cresol and p-cresol.

The conditions selected for carrying out this hydroxymethylation stepare those recommended by the prior art listed below: cf in particular H.G. PEER, Rec. Trav. Chim. Netherlands 79, 825-835 (1960); GB-A-774 696;GB-A-751 845; EP-A-165; J. H. FREEMAN, J. Am. Chem. Soc. 74, 6 257-6 260(1952); and 76 2080-2087 (1954); H. G. PEER, Rec. Trav. Chim.Netherlands 78 851-863 (1959); H. EULER et al., Arkiv für Chem. 13, 1-7(1939); P. CLAUS et al., Monath. Chem. 103, 1178-11293 (1972).

Formaldehyde or any formaldehyde generator can be used, such as trioxaneor paraformaldehyde used as linear paraformaldehydes of any degree ofpolymerisation, preferably containing 8 to 100 (CH₂O) units.

Formaldehyde can be used as an aqueous solution in a concentration whichis not critical. It can be in the range 20% to 50% by weight:preferably, commercial solutions are used which have a concentration ofabout 30% to 40% by weight.

The quantity of formaldehyde, expressed as moles of formaldehyde permole of phenol, can vary between wide limits. The formaldehyde/phenolmolar ratio can be between 1.0 and 4.0, preferably between 1.0 and 2.5.

The quantity of base present in the hydroxymethylation medium, expressedas the number of moles of base/phenolic hydroxy group of the phenol tobe hydroxymethylated, can vary between wide limits. In general, thisratio, which varies depending on the base, can be between 1.0 and 4.0,preferably between 0.9 and 2.0. The base used may be one of those citedabove for the oxidation step. Aqueous solutions of alkaline hydroxidesare particularly suitable.

In general, the hydroxymethylation step is carried out at a temperaturein the range 0° C. to 100° C., preferably in the range 20° C. to 70° C.

The process is preferably carried out at a pressure which is autogenousfor the reactants to avoid any paraformaldehyde losses which may begaseous at the temperatures used.

Preferably, the reaction is carried out in a controlled atmosphere ofinert gas such as nitrogen or a noble gas, for example argon.

The reaction time can readily be determined by the skilled persondepending on the desired degree of conversion of the starting phenol andtaking into account the necessity to minimise by-products such asbis-arylmethane. It is usually between 15 minutes and 4 hours,preferably between 1 hour and 3 hours.

The degree of conversion of phenol is controlled by different parameters(temperature, duration, quantity of reactants). It is advantageously inthe range 60% to 95%, preferably in the range 80% to 95%.

In practice, the reaction is readily carried out by charging the phenoland formaldehyde, and any base into the apparatus, then stirring andheating the reaction mixture to the desired temperature for the timerequired to complete the reaction.

The order of introduction of the reactants is not critical and can thusbe different.

A mixture of phenolic compounds with formula (IIa₁) to (IIc₁) isobtained.

Compounds with formula (IIa₂) to (IIc₂) can be prepared by oxidisinghydroxymethylated phenolic compounds with formula (IIa₁) to (IIc₁), byoxidising using molecular oxygen or a gas containing molecular oxygen,in an aqueous alkaline phase, in the presence of a catalyst based on ametal from group 8 of the periodic table, preferably platinum orpalladium, optionally containing metals such as cadmium, cerium,bismuth, lead, silver, tellurium or tin as an activator. Such processeshave been described in U.S. Pat. No. 3,673,257, FR-A-2 305 420 andFR-A-2 350 323.

If necessary, the pH of the solution can be raised to a value in therange 8 to 13 by optional addition of an alkaline or alkaline-earthbase. The optional value of the pH depends on the nature of thehydroxymethylated phenols.

The temperature of the oxidation reaction is between 10° C. and 100° C.,preferably between 20° C. and 60° C.

More specifically again, the process of the present invention issuitable for the preparation of compounds with formulae (IIa₂) to (IIc₂)from phenolic compounds with formulae (IIa₁) to (IIc₁) resulting fromthe first step, using molecular oxygen or a gas containing molecularoxygen, in the presence of a catalyst based on a metal from group 8 ofthe periodic table, optionally containing a metal such as those used asan activator, without intermediate separation of the hydroxymethylatedphenolic compounds.

From an industrial viewpoint, it is particularly advantageous whencarrying out the process of the present invention to use compounds withformula (IIa₂) to (IIc₂) obtained by a two-step process comprising:

hydroxymethylation of a phenol in an aqueous medium in the presence ofan alkaline or alkaline-earth base using formaldehyde or a formaldehydegenerator, resulting in a mixture of hydroxymethylated phenoliccompounds, one being hydroxymethylated in the 2 and 4 positions and thetwo others being hydroxymethylated in the 2 or in the 4 position;

and oxidation, without intermediate separation, of the phenoliccompounds obtained using molecular oxygen or a gas containing molecularoxygen, in an aqueous alkaline phase in the presence of a catalyst basedon a metal from group 8 of the periodic table, and optionally, a metalsuch as those cited above, acting as an activator.

An additional advantage of the process of the invention is that itallows mixtures of phenolic compounds issuing directly from thepreceding hydroxymethylation and optional oxidation steps to be used.

As mentioned above, the process of the invention is particularlysuitable for the preparation of vanillin and ethylvanillin from amixture of phenolic compounds obtained by hydroxymethylation of guaiacolor guetol.

Thus vanillin can be prepared by selectively oxidising a mixture ofphenolic compounds, 4,6-di(hydroxymethyl)guaiacol (A),p-hydroxymethylguaiacol (B) and o-hydroxymethylguaiacol (C) at thehydroxymethyl group in the 2-position of compounds (A) and (C) to acarboxy group, and the hydroxymethyl group in the 4 position incompounds (A) and (B) to a formyl group, resulting in a mixture of3-carboxy-4-hydroxy-5-methoxybenzaldehyde, vanillin and2-hydroxy-3-methoxybenzoic acid which, after decarboxylation, producesvanillin and guaiacol which can be recycled.

A variation consists of selectively oxidising a mixture of phenoliccompounds, 4,6-di(formyl)guaiacol (A), p-formylguaiacol (B) ando-formylguaiacol (C) at the formyl group in the 2-position of compounds(A) and (C) to a carboxy group, resulting in a mixture of3-carboxy-4-hydroxy-5-methoxybenzaldehyde, vanillin and2-hydroxy-3-methoxybenzoic acid which, after decarboxylation, producesvanillin and guaiacol which can be recycled.

Ethylvanillin is prepared by the process of the present invention byselectively oxidising a mixture of phenolic compounds,4,6-di(hydroxymethyl)guetol (A), p-hydroxymethylguetol (B) ando-hydroxymethylguetol (C) at the hydroxymethyl group in the 2-positionof compounds (A) and (C) to a carboxy group, and the hydroxymethyl groupin the 4 position in compounds (A) and (B) to a formyl group, resultingin a mixture of 3-carboxy-4-hydroxy-5-ethoxybenzaldehyde, ethylvanillinand 2-hydroxy-3-ethoxybenzoic acid which, after decarboxylation,produces ethylvanillin and guetol which can be recycled.

A further variation consists of selectively oxidising a mixture ofphenolic compounds, 4,6-di(formyl)guetol (A), p-formylguetol (B) ando-formylguetol (C) at the formyl group in the 2-position of compounds(A) and (C) to a carboxy group, resulting in a mixture of3-carboxy-4-hydroxy-5-ethoxybenzaldehyde, ethylvanillin and2-hydroxy-3-ethoxybenzoic acid which, after decarboxylation, producesvanillin and guaiacol which can be recycled.

Examples of implementations of the invention will now be given. Theseexamples are given by way of indication and are in no way limiting.

The degree of conversion and the yield obtained are defined in theexamples.

The degree of conversion (TT) corresponds to the ratio between thenumber of moles of substrate transformed and the number of moles ofsubstrate used.

The yield (RR) corresponds to the ratio between the number of moles ofproduct formed and the number of moles of substrate used.

The yield (RT_(vanillin)) corresponds to the ratio between the number ofmoles of vanillin formed and the number of moles of guaiacol transformedin the sequence.

The following abbreviations are used in the examples:

o-hydroxymethylguaiacol=OMG

p-hydroxymethylguaiacol=PMG

o-vanillin=3-methoxy-2-hydroxybenzaldehyde=OVA

p-vanillin=3-methoxy-4-hydroxybenzaldehyde=PVA

o-vanillic acid=2-hydroxy-3-methoxybenzoic acid=AOV

p-vanillic acid=4-hydroxy-3-methoxybenzoic acid=APV

4,6-di(hydroxymethyl) guaiacol=DMG

4,3-(diformyl)guaiacol=DFG

o-carboxyvanillin=OCVA

4,6-(dicarboxy)guaiacol=DCG.

EXAMPLE 1

1. Condensation Step

The following was charged into a 2 liter reactor provided with amechanical stirrer and a temperature regulation means:

152 g of guaiacol;

249 g of an aqueous 30% formol solution;

49.2 g of caustic soda;

872 g of water.

The reaction medium was kept at 45° C. for 1 hour then cooled andanalysed using high performance liquid chromatography.

The reaction balance was as follows:

guaiacol conversion=90%

o-hydroxymethyl guaiacol (OMG) yield=15%

p-hydroxymethyl guaiacol (PMG) yield=18%

4,6-di(hydroxymethyl) guaiacol (DMG) yield=83%

The sum of the upgradable products was 93%.

2. Oxidation Step

The reaction medium was then diluted with 1500 g of water and 148 g ofcaustic soda.

The reaction medium was then introduced into a 3.9 liter autoclaveprovided with a self exhausting turbine.

0.54 g of bismuth trioxide and 22 g of a palladium catalyst deposited oncarbon black in a proportion of 3% by weight of metal were then added.

Stirring was started at a rate of 1500 rpm and the temperature of thereaction medium was raised to 45° C. under nitrogen. It was then placedunder a pressure of 3 bars and air was introduced into the reactionmedium at a rate of 300 g/h. The reaction medium was held under theseconditions for 6 hours.

The reaction medium was cooled and the pressure was reduced toatmospheric pressure then the catalyst was filtered.

The reaction medium was then analysed by high performance liquidchromatography.

The yields were as follows (for the complete sequence):

TT guaiacol=92%

ortho series

RR o-hydroxymethyl guaiacol (OMG)=0%

RR orthovanillin (OVA)=1%

RR orthovanillic acid (AOV)=14%

para series

RR p-hydroxymethylguaiacol (PMG)=0%

RR vanillin (PVA)=16%

RR para-vanillic acid (APV)=1%

di series

RR 4,6-di(hydroxymethyl) guaiacol (DMG)=0%

RR 4,6-(diformyl) guaiacol (DFG)=1%

RR orthocarboxyvanillin (OCVA)=47%

RR 4,6-(dicarboxy) guaiacol (DCG)=10%

The sum of the yields of the upgradable products(guaiacol+OAV+PVA+APV+OCVA+DCG) was 87%.

3. Decarboxylation of Reaction Mixture

199.91 g of this reaction mixture was neutralised with 16.69 g of 92%sulphuric acid and introduced into a 300 ml autoclave provided with aturbine and a temperature regulation system.

The reaction medium was heated to 175° C. for 3 hours under autogenouspressure then cooled and measured using liquid chromatography.

The vanillin yield and the guaiacol conversion were as follows:

TT guaiacol=76%/initial guaiacol

RR vanillin=61%/initial guaiacol, i.e., RT vanillin=80%

EXAMPLE 2

1. Condensation Step

The following was charged into a 2 liter reactor provided with amechanical stirrer and a temperature regulation means:

133 g of guaiacol;

202 g of an aqueous 30% formol solution;

145 g of an aqueous 30% caustic soda solution;

480 g of water.

The reaction medium was kept at 47° C. for 0 h 50 then cooled 290 g ofan aqueous 30% caustic soda solution was added. It was analysed usinghigh performance liquid chromatography. The reaction balance was asfollows:

guaiacol conversion=97%

o-hydroxymethyl guaiacol (OMG) yield=10%

p-hydroxymethyl guaiacol (PMG) yield=12%

4,6-di(hydroxymethyl) guaiacol (DMG) yield=70%

(OMG+DMG+PMG) yields=92%

The sum of the RT yields of the upgradable products was 95%.

2. Oxidation Step

The reaction medium was then diluted with 1230 g of water

The reaction medium was introduced into a 3.9 liter autoclave providedwith a self exhausting turbine.

0.54 g of bismuth trioxide and 34.5 g of a platinum catalyst depositedon carbon black in a proportion of 2% by weight of metal were thenadded.

Stirring was started at a rate of 950 rpm and the temperature of thereaction medium was raised to 70° C. under nitrogen. It was then placedunder a pressure of 4 bars and air was introduced into the reactionmedium at a rate of 200 g/h. The reaction medium was held under theseconditions for 5 hours.

The reaction medium was cooled and the pressure was reduced toatmospheric pressure then the catalyst was filtered.

The reaction medium was then analysed by high performance liquidchromatography.

The yields were as follows (for the complete sequence):

TT guaiacol=97%

ortho series

RR o-hydroxymethyl guaiacol (OMG)=0%

RR orthovanillin (OVA)=1%

RR orthovanillic acid (AOV)=6%

para series

RR p-hydroxymethylguaiacol (PMG)=0%

RR vanillin (PVA)=9%

RR para-vanillic acid (APV)=2%

di series

RR 4,6-di(hydroxymethyl) guaiacol (DMG)=0%

RR 4,6-(diformyl) guaiacol (DFG)=1%

RR orthocarboxyvanillin (OCVA)=54%

RR 4,6-(dicarboxy) guaiacol (DCG)=6%.

The sum of the yields of the upgradable products(guaiacol+OAV+PVA+APV+OCVA+DCG) was 80%.

3. Decarboxylation of Reaction Mixture

150 g of this reaction mixture was neutralised with 15 ml of 10 mol/lsulphuric acid and introduced into a 300 ml autoclave provided with aturbine and a temperature regulation system.

The reaction medium was heated to 175° C. for 2 hours under autogenouspressure then cooled and measured using liquid chromatography.

The vanillin yield and the guaiacol conversion were as follows:

TT guaiacol=91%/initial guaiacol

RR vanillin=52%/initial guaiacol, i.e., RT vanillin=57%

What is claimed is:
 1. A process for the preparation of a4-hydroxybenzaldehyde and its derivatives, comprising the steps of: a)selectively oxidizing, to a carboxy group, the group in the 2 positionwith respect to the hydroxyl group, present in a starting mixture ofphenolic compounds comprising at least: a phenolic compound (A) carryingformyl and/or hydroxymethyl groups in the 2 and 4 position; a phenolicgroup (B) carrying a formyl or hydroxymethyl group in the 4 position;and a phenolic compound (C) carrying a formyl or hydroxymethyl group inthe 2 position; resulting in a mixture comprising a3-carboxy-4-hydroxybenzaldehyde, a 4-hydroxybenzaldehyde and a2-hydroxybenzoic acid, b) carrying out a decarboxylation operation onsaid resulting mixture obtained in step a) to produce the4-hydroxybenzaldehyde and a phenol, and c) optionally, recycling thephenol obtained in step b).
 2. A process according to claim 1, whereinthe mixture of phenolic compounds of step a) has general formula (II):

wherein, in formulae (IIA) to (IIC): Y₁ and Y₂, which are identical ordifferent, represent a —CHO group or a —CH₂OH group; Z₁, Z₂ and Z₃,which are identical or different, represent a hydrogen atom, an alkyl,alkenyl, alkoxy, hydroxyalkyl, alkoxyalkyl, cycloalkyl, aryl radical, ahydroxy group, a nitro group, a halogen atom, or a trifluoromethylgroup.
 3. A process according to claim 2, wherein the phenolic compoundshave formulae (IIA) to (IIC) wherein Z₁, Z₂ and Z₃, which are identicalor different, represent a hydrogen atom, a linear alkyl radicalcontaining 1 to 12 carbon atoms, a branched alkyl radical containing 1to 12 carbon atoms, a linear alkenyl radical containing 2 to 12 carbonatoms, a branched alkenyl radical containing 2 to 12 carbon atoms, alinear alkoxy radical containing 1 to 12 carbon atoms, a branched alkoxyradical containing 1 to 12 carbon atoms, a phenyl radical, or a halogenatom.
 4. A process according to claim 3, wherein Z₁, Z₂ and Z₃, whichare identical or different, represent methyl, ethyl, propyl, isopropyl,butyl, isobutyl, sec-butyl, tert-butyl, vinyl, allyl, methoxy, ethoxy,propoxy, isopropoxy, butoxy, isobutoxy, sec-butoxy, tert-butoxy,fluorine, chlorine or bromine atom.
 5. A process according to claim 2,wherein the phenolic compounds have formulae (IIA) to (IIC), wherein Z₁represents a linear alkyl, a branched alkyl or an alkoxy radicalcontaining 1 to 6 carbon atoms; Z₂ and Z₃ represent a hydrogen atom; andY₁ and Y₂ are identical and represent a formyl group or a hydroxymethylgroup.
 6. A process according to claim 2, wherein the mixture ofphenolic compounds with formula (II) is: o-hydroxymethylguaiacol,p-hydroxymethylguaiacol and 4,6-di(hydroxymethyl)guaiacol;o-formylguaiacol, p-formylguaiacol and 4,6-diformylguaiacol;o-hydroxymethylguetol, p-hydroxymethylguetol and4,6-di(hydroxymethyl)guetol; and o-formylguetol, p-formylguetol and4,6-diformylguetol.
 7. A process according to claim 2, wherein in stepa), the mixture of phenolic compounds with formula (II) is oxidized inthe liquid phase using molecular oxygen or a gas containing molecularoxygen, in an aqueous medium comprising a basic agent, in the presenceof a catalyst comprising a metal M₁ from group 1b and 8 of the periodicclassification of the elements, said catalyst optionally containing, asan activator, a metal selected from the group consisting of cadmium,cerium, bismuth, lead, silver, tellurium and tin.
 8. A process accordingto claim 7, wherein the catalyst is copper, nickel, ruthenium, rhodium,palladium, osmium, iridium, or platinum.
 9. A process according to claim8, wherein the catalyst is platinum or palladium in the form of platinumblack, palladium black, platinum oxide, palladium oxide or platinum orpalladium itself deposited on a support.
 10. A process according toclaim 8, wherein the support. is carbon black, calcium carbonate,aluminas or activated silicas.
 11. A process according to claim 7,wherein the quantity of catalyst used, expressed as the weight of metalM₁ with respect to that of the phenolic compound with formula (II), isfrom 0.01% to 10%.
 12. A process according to claim 7, wherein thecatalyst further comprises an activator which is an organic or inorganicbismuth derivative selected from the group consisting of bismuth oxides;bismuth hydroxides; bismuth or bismuthyl salts of inorganic hydracids,bismuth or bismuthyl salts of inorganic oxyacids, bismuth or bismuthylsalts of aliphatic or aromatic organic acids; and bismuth or bismuthylphenates.
 13. A process according to claim 12, wherein the bismuthderivative is bismuth oxide Bi₂O₃, bismuth oxide Bi₂O₄; bismuthhydroxide Bi(OH)₃; bismuth chloride BiCl₃; bismuth bromide BiBr₃;bismuth iodide BiI₃; neutral bismuth sulphate Bi₂(SO₄)₃; neutral bismuthnitrate Bi(NO₃)₃,5H₂O; bismuthyl nitrate BiO(NO₃); bismuthyl carbonate(BiO)₂CO₃,0.5H₂O; bismuth acetate Bi(C₂H₃O₂)₃ or bismuth salicylateC₆H₄CO₂(BiO)OH.
 14. A process according to claim 13, wherein thequantity of activator is selected so that the medium contains at least0.1% by weight of metal activator with respect to the weight of metal M₁used, and 10 to 900 ppm by weight of metal M₁ with respect to themixture of phenolic compounds with formula (II).
 15. A process accordingto claim 7, wherein step a) is carried out within a temperature range of30° C. to 200° C.
 16. A process according to claim 15, wherein step a)is carried out within a temperature range of 40° C. and 160° C.
 17. Aprocess according to claim 7, wherein step a) is carried out at apressure of 1 to 20 bar.
 18. A process according to claim 7, whereinstep a) is carried out in an aqueous medium containing, in solution, abasic agent in a quantity such that it represents 0.5 to 10 moles ofbasic agent per mole of phenolic compounds with formula (II).
 19. Aprocess according to claim 18, wherein the basic agent is sodiumhydroxide or potassium hydroxide in a quantity such that it represents 2to 4 moles, of basic agent per mole of phenolic compounds with formula(II).
 20. A process according to claim 18, wherein step a) is carriedout with a platinum catalyst at a temperature of between 60° C. and 160°C.; the quantity of base being in the range 1 to 3 moles per mole ofphenolic compounds with formula (II).
 21. A process according to claim18, wherein step a) is carried out with a palladium catalyst at atemperature of between 30° C. and 200° C., the quantity of base being inthe range 2 to 4 moles per mole of phenolic compounds with formula (II).22. A process according to claim 1, wherein the reaction mixtureobtained in step a) comprises a 3-carboxy-4-hydroxybenzaldehyde, a2-hydroxybenzoic acid, and a 4-hydroxybenzaldehyde which are completelyor partially in their salt form.
 23. A process according to claim 1,wherein the mixture to be decarboxylated in step b) comprises acids ofthe following formula:

wherein, in said formulae (III) and (IV): M represents a hydrogen atom,a metallic cation from group (Ia) or (IIa), or an ammonium cation; andZ₁, Z₂ and Z₃, which are identical or different, represent a hydrogenatom, an alkyl, alkenyl, alkoxy, hydroxyalkyl, alkoxyalkyl, cycloalkyl,aryl radical, a hydroxy group, a nitro group, a halogen atom, or atrifluoromethyl group.
 24. A process according to claim 23, wherein saidacid is decarboxylated by addition of a protonic inorganic or organicacid, until a pH of no more than 3 is obtained.
 25. A process accordingto claim 24, wherein step b), further comprising heating the mixture tobe decarboxylated to a temperature of between 120° C. and 350° C.,cooling said mixture, and separating the 4-hydroxybenzaldehyde which hasthe following formula (VI):

in which formula (VI): Z₁, Z₂ and Z₃ which are identical or different,represent a hydrogen atom, an alkyl, alkenyl, alkoxy, hydroxyalkyl,alkoxyalkyl, cycloalkyl, aryl radical, a hydroxy group, a nitro group, ahalogen atom, or a trifluoromethyl group.
 26. A mixture of phenoliccompounds which are completely or partially in their salt form,comprising: a phenolic compound (A) carrying formyl or hydroxymethylgroups in the 2 and 4 position; a phenolic compound (B) carrying aformyl group or a hydroxymethyl group in the 4 position; and a phenoliccompound (C) carrying a formyl or a hydroxymethyl group in the 2position.
 27. A mixture of phenolic compounds having general formula(II):

wherein: M represents a hydrogen atom, a metallic cation from group (Ia)or (IIa), or an ammonium cation; and Z₁, Z₂ and Z₃ which are identicalor different, represent a hydrogen atom, an alkyl, alkenyl, alkoxy,hydroxyalkyl, alkoxyalkyl, cycloalkyl, aryl radical, a hydroxy group, anitro group, a halogen atom, or a trifluoromethyl group.
 28. A mixtureof phenolic compounds having general formula (II′):

in which formulae: M represents a hydrogen atom, a metallic cation fromgroup (Ia) or (IIa), or an ammonium cation; and Z₁, Z₂ and Z₃ which areidentical or different, represent a hydrogen atom, an alkyl, alkenyl,alkoxy, hydroxyalkyl, alkoxyalkyl, cycloalkyl, aryl radical, a hydroxygroup, a nitro group, a halogen atom, or a trifluoromethyl group.
 29. Amixture of phenolic compounds according to claim 26, comprises: 30% to70% by weight, of the phenolic compound (A); 30% to 70% of a mixture ofphenolic compounds (B+C).
 30. A process for the preparation of a mixtureof phenolic compounds as defined in claim 26, comprising the stepsof: 1) hydroxymethylating a phenol by condensing it with formaldehyde ora formaldehyde generator in an aqueous phase in the presence of analkaline or alkaline-earth base in order to obtain a degree ofconversion of phenol of at most 95%, and, then, 2) optionally, carryingout an oxidation step.
 31. A process according to claim 30, wherein thedegree of phenol conversion is 80% to 95%.
 32. A process according toclaim 30, wherein the starting phenol is a phenol which is notsubstituted in the ortho and para positions with respect to the hydroxylgroup, with general formula (I):

wherein Z₁, Z₂ and Z₃ which are identical or different, represent ahydrogen atom, an alkyl, alkenyl, alkoxy, hydroxyalkyl, alkoxyalkyl,cycloalkyl, aryl radical, a hydroxy group, a nitro group, a halogenatom, or a trifluoromethyl group.
 33. A process according to claim 32,wherein the phenol with formula (I) is guaiacol, guetol,3-methoxyphenol, 3-ethoxyphenol, 3-isopropoxyphenol, 3-t-butoxyphenol,m-cresol, or o-cresol.
 34. A process according to claim 30, whereinformaldehyde or formaldehyde generator is trioxane or para-formaldehyde,in the form of a linear polyformaldehyde containing 8 to 100 (CH₂O)units.
 35. A process according to claim 34, wherein step 1) is carriedout with a formaldehyde/phenol molar ratio of between 1.0 and 4.0.
 36. Aprocess according to claim 30, wherein the quantity of base present inthe hydroxymethylation medium, expressed as a number of moles ofbase/phenolic hydroxy group of the phenol to be hydroxymethylated, isbetween 1.0 and 4.0.
 37. A process according to claim 30, wherein thehydroxymethylation is carried out at a temperature in the range 0° C. to100° C.
 38. A process according to claim 30, wherein thehydroxymethylation is carried out between 15 minutes and 4 hours.
 39. Aprocess for the preparation of a mixture of compounds with formula(IIa₂) to (IIc₂):

wherein: M represents a hydrogen atom, a metallic cation from group (Ia)or (IIa), or an ammonium cation; and Z₁, Z₂ and Z₃ which are identicalor different, represent a hydrogen atom, an alkyl, alkenyl, alkoxy,hydroxyalkyl, alkoxyalkyl, cycloalkyl, aryl radical, a hydroxy group, anitro group, a halogen atom, or a trifluoromethyl group, said processcomprising the steps of: a) oxidizing hydroxymethylated phenoliccompounds with formula (IIa₁) to (IIc₁)

in which formulae: M represents a hydrogen atom, a metallic cation fromgroup (Ia) or (IIa), or an ammonium cation; and Z₁, Z₂ and Z₃ which areidentical or different, represent a hydrogen atom, an alkyl, alkenyl,alkoxy, hydroxyalkyl, alkoxyalkyl, cycloalkyl, aryl radical, a hydroxygroup, a nitro group, a halogen atom, or a trifluoromethyl group, byusing molecular oxygen or a gas containing molecular oxygen, in anaqueous alkaline phase in the presence of a catalyst which is a metalfrom group 8 of the periodic table, and optionally containing cadmium,cerium, bismuth, lead, silver, tellurium or tin as an activator.
 40. Aprocess according to claim 39, wherein the pH of the solution is broughtto a value in the range 8 to 13 by optional addition of an alkaline oralkaline-earth base.
 41. A process according to claim 39, wherein thetemperature of the oxidation reaction is in the range 10° C. to 100° C.42. A process according to claim 39, wherein the mixture of phenoliccompounds with formula (Ila₂) to (IIc₂) is obtained by a two-stepprocess comprising: hydroxymethylation of a phenol in an aqueous mediumin the presence of an alkaline or alkaline-earth base using formaldehydeor a formaldehyde generator resulting in a mixture of hydroxymethylatedphenolic compounds, one being hydroxymethylated in the 2 and 4 positionsand the two others being hydroxymethylated in the 2 or in the 4position; and oxidation, without intermediate separation, of thephenolic compounds obtained using molecular oxygen or a gas containingmolecular oxygen, in an aqueous alkaline phase in the presence of acatalyst which is a metal from group 8 of the periodic table, andoptionally, a metal acting as an activator.
 43. A process for thepreparation of vanillin according to claim 1, wherein a mixture ofphenolic compounds, 4,6-di(hydroxymethyl)guaiacol (A),p-hydroxymethylguaiacol (B) and o-hydroxymethylguaiacol (C) isselectively oxidised at the hydroxymethyl group in the 2-position ofcompounds (A) and (C) to a carboxy group, and at the hydroxymethyl groupin the 4 position in compounds (A) and (B) to a formyl group, resultingin a mixture of 3-carboxy-4-hydroxy-5-methoxybenzaldehyde, vanillin and2-hydroxy-3-methoxybenzoic acid which, after decarboxylation, producesvanillin and guaiacol which can be recycled.
 44. A process for thepreparation of vanillin according to claim 1, wherein a mixture ofphenolic compounds, 4,6-di(formyl)guaiacol (A), p-formylguaiacol (B) ando-formylguaiacol (C) is selectively oxidised at the formyl group in the2-position of compounds (A) and (C) to a carboxy group, resulting in amixture of 3-carboxy-4-hydroxy-5-methoxybenzaldehyde, vanillin and2-hydroxy-3-methoxybenzoic acid which, after decarboxylation, producesvanillin and guaiacol which can be recycled.
 45. A process for thepreparation of ethylvanillin according to claim 1, wherein a mixture ofphenolic compounds, 4,6-di(hydroxymethyl)guetol (A),p-hydroxymethylguetol (B) and o-hydroxymethylguetol (C) is selectivelyoxidized at the hydroxymethyl group in the 2-position of compounds (A)and (C) to a carboxy group, and at the hydroxymethyl group in the 4position in compounds (A) and (B) to a formyl group, resulting in amixture of 3-carboxy-4-hydroxy-5-ethoxybenzaldehyde, ethylvanillin and2-hydroxy-3-ethoxybenzoic acid which, after decarboxylation, producesethylvanillin and guetol which can be recycled.
 46. A process for thepreparation of ethylvanillin according to claim 1, wherein a mixture ofphenolic compounds, 4,6-di(formyl)guetol (A), p-formylguetol (B) ando-formylguetol (C) is selectively oxidized at the formyl group in the2-position of compounds (A) and (C) to a carboxy group, resulting in amixture of 3-carboxy-4-hydroxy-5-ethoxybenzaldehyde, ethylvanillin and2-hydroxy-3-ethoxybenzoic acid which, after decarboxylation, producesethylvanillin and guetol which can be recycled.